You know, you could always just have the God of Magic revoke his Diviner license, as it's pretty obvious he's fraudulently passing himself off as a Diviner. There's got to be rules and regulations.
In my campaign, Diviners get inspected every year by the Diviner's Guild. Nothing terribly difficult, just a little "strip the guy naked, put a blindfold on him, spin him around, and make him walk through a room filled with Glyphs of Warding." If his communes work out ok, then he passes just fine. If not, oh well then, who the hell really cares about Diviners?
Some of the NPCs in the Diviner's Guild are seeking to reduce the risk involved in Diviner inspections by just hiding their keys and seeing if they find them or not, but I think that'd be pretty wussy.
Edit: I meant Glyphs of Warding. Actually, I meant Delayed Blast Fireballs, but Glyphs of Warding make more sense.